Messages in this thread Patch in this message | | | Date | Fri, 6 Dec 2019 16:31:29 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] futex: Support smaller futexes of one byte or two byte size. |
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On Wed, Dec 04, 2019 at 06:52:38PM -0500, Malte Skarupke wrote: > Two reasons for adding this: > 1. People often want small mutexes. Having mutexes that are only one byte > in size was the first reason WebKit mentioned for re-implementing futexes > in a library: > https://webkit.org/blog/6161/locking-in-webkit/
They have the best locking namespace _ever_!! Most impressed with them for getting away with that.
> I had to change where we store two flags that were previously stored in > the low bits of the address, since the address can no longer be assumed > to be aligned on four bytes. Luckily the high bits were free as long as > PAGE_SIZE is never more than 1 gigabyte.
For now it seems unlikely to run with 1G base pages :-)
> The reason for only supporting u8 and u16 is that those were easy to > support with the existing interface. 64 bit futexes would require bigger > changes.
Might make sense to enumerate the issues you've found that stand in the way of 64bit futexes. But yes, I can think of a few.
> @@ -467,7 +469,14 @@ static void drop_futex_key_refs(union futex_key *key) > > enum futex_access {
I prefer FUTEX_NONE, to match PROT_NONE.
> FUTEX_READ, > - FUTEX_WRITE > + FUTEX_WRITE, > + /* > + * for operations that only need the address and don't touch the > + * memory, like FUTEX_WAKE or FUTEX_REQUEUE. (not FUTEX_CMP_REQUEUE) > + * this will skip the size checks of the futex, allowing those > + * operations to be used with futexes of any size. > + */ > + FUTEX_NO_READ_WRITE > }; > > /**
> @@ -520,25 +529,37 @@ futex_setup_timer(ktime_t *time, struct hrtimer_sleeper *timeout, > * lock_page() might sleep, the caller should not hold a spinlock. > */ > static int > -get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, enum futex_access rw) > +get_futex_key(u32 __user *uaddr, int flags, union futex_key *key, enum futex_access rw) > { > unsigned long address = (unsigned long)uaddr; > struct mm_struct *mm = current->mm; > struct page *page, *tail; > struct address_space *mapping; > - int err, ro = 0; > + int err, fshared, ro = 0; > + > + fshared = flags & FLAGS_SHARED; > > /* > * The futex address must be "naturally" aligned. > */ > + if (flags & FLAGS_8_BITS || rw == FUTEX_NO_READ_WRITE) { > + if (unlikely(!access_ok(uaddr, sizeof(u8)))) > + return -EFAULT; > + } else if (flags & FLAGS_16_BITS) { > + if (unlikely((address % sizeof(u16)) != 0)) > + return -EINVAL; > + if (unlikely(!access_ok(uaddr, sizeof(u16)))) > + return -EFAULT; > + } else { > + if (unlikely((address % sizeof(u32)) != 0)) > + return -EINVAL; > + if (unlikely(!access_ok(uaddr, sizeof(u32)))) > + return -EFAULT; > + }
That might be better written like:
if (rw != FUTEX_NONE) size = futex_size(flags); else size = 1;
if (unlikely((address % size) != 0)) return -EINVAL; if (unlikely(!access_ok(uaddr, size))) return -EFAULT;
> @@ -799,6 +820,48 @@ static int get_futex_value_locked(u32 *dest, u32 __user *from) > return ret ? -EFAULT : 0; > } > > +static int > +get_futex_value_locked_any_size(u32 *dest, u32 __user *from, int flags) > +{ > + int ret; > + u8 dest_8_bits; > + u16 dest_16_bits; > + > + pagefault_disable(); > + if (flags & FLAGS_8_BITS) { > + ret = __get_user(dest_8_bits, (u8 __user *)from); > + *dest = dest_8_bits; > + } else if (flags & FLAGS_16_BITS) { > + ret = __get_user(dest_16_bits, (u16 __user *)from); > + *dest = dest_16_bits; > + } else { > + ret = __get_user(*dest, from); > + } > + pagefault_enable(); > + > + return ret ? -EFAULT : 0; > +} > + > +static int get_futex_value_any_size(u32 *dest, u32 __user *from, int flags) > +{ > + int ret; > + u8 uval_8_bits; > + u16 uval_16_bits; > + > + if (flags & FLAGS_8_BITS) { > + ret = get_user(uval_8_bits, (u8 __user *)from); > + if (ret == 0) > + *dest = uval_8_bits; > + } else if (flags & FLAGS_16_BITS) { > + ret = get_user(uval_16_bits, (u16 __user *)from); > + if (ret == 0) > + *dest = uval_16_bits; > + } else { > + ret = get_user(*dest, from); > + } > + return ret; > +}
Perhaps write that like:
static inline int __get_futex_value(u32 *dest, u32 __user *from, int flags) { u32 val = 0; int ret;
switch (futex_size(flags)) { case 1: /* * afaict __get_user() doesn't care about the type of * the first argument */ ret = __get_user(val, (u8 __user *)from); break;
case 2: ....
case 4:
} if (!ret) *dest = val; return ret; } static inline int get_futex_value(...) { do uaccess check return __get_futex_value(); } static inline int get_futex_value_locked(...) { int ret; pagefault_disable(); ret = __get_futex_value(...); pagefault_enable();
return ret; } > /* > * PI code:
> + if (op & FUTEX_ALL_SIZE_BITS) { > + switch (cmd) { > + case FUTEX_CMP_REQUEUE: > + /* > + * for cmp_requeue, only uaddr has to be of the size > + * passed in the flags. uaddr2 can be of any size > + */ > + case FUTEX_WAIT: > + case FUTEX_WAIT_BITSET: > + switch (op & FUTEX_ALL_SIZE_BITS) { > + case FUTEX_SIZE_8_BITS: > + flags |= FLAGS_8_BITS; > + break; > + case FUTEX_SIZE_16_BITS: > + flags |= FLAGS_16_BITS; > + break; > + case FUTEX_SIZE_32_BITS: > + break; > + default: > + /* > + * if both bits are set we could silently treat > + * that as a 32 bit futex, but we may want to > + * use this pattern in the future to indicate a > + * 64 bit futex or an arbitrarily sized futex. > + * so we don't want code relying on this yet. > + */ > + return -EINVAL; > + } > + break; > + case FUTEX_WAKE: > + case FUTEX_REQUEUE: > + /* > + * these instructions work with sized mutexes, but you > + * don't need to pass the size. we could silently > + * ignore the size argument, but the code won't verify > + * that the correct size is used, so it's preferable > + * to make that clear to the caller. > + * > + * for requeue the meaning would also be ambiguous: do > + * both of them have to be the same size or not? they > + * don't, and that's clearer when you just don't pass > + * a size argument. > + */ > + return -EINVAL;
Took me a while to figure out this relies on FUTEX_NONE to avoid the alignment tests.
> + default: > + /* > + * all other cases are not supported yet > + */ > + return -EINVAL; > + } > + } > > switch (cmd) { > case FUTEX_LOCK_PI: > -- > 2.17.1 >
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